PROJECT SUMMARY/ABSTRACT Oxygen is one of the most used substrates in the human body. When oxygen deprivation exceeds the buffering capacity of the human body, there are devastating effects on health and survival. For example, three of the five leading causes of death in the US are a consequence of impaired oxygenation ? heart disease, respiratory disease and stroke. Indeed, over 400,000 individuals suffer from a stroke each year in the US alone, leaving a great unmet need for new therapies. By uncovering how tissues sense and adapt to variations in oxygen tensions, we can better understand and treat such conditions of impaired oxygenation. The mitochondrial electron transport chain (ETC) consumes 90% of the body's oxygen, while providing 90% of the ATP supply. Interestingly, the reliance on the ETC for energy production varies substantially across tissues. The remaining oxygen consumption arises from several hundred oxygen-dependent reactions that also occur in a highly tissue-specific manner. Moreover, hypoxia tolerance varies drastically across different tissues. At one extreme, the brain can only survive for several minutes without oxygen. At the other extreme, skeletal muscle can survive several hours of anoxia without permanent damage. This wide range of metabolic flexibilities across the human body serves as a fascinating and useful tool to study adaptive mechanisms for hypoxia. Traditionally, comparative physiologists have drawn inspiration from extreme organisms (e.g. painted turtles, Weddell seals) that can survive without oxygen for hours or days at a time. However, these strategies are rarely translatable as humans do not possess the unique metabolic pathways or physiology of these organisms. Instead, I propose a modern twist to a classical problem ? the use of comparative metabolism across the most extreme tissues to identify oxygen sensing and adaptive pathways. More specifically, I propose varying oxygen tensions and (Aim 1) comparing the bioenergetics and metabolism between primary neurons vs. skeletal myotubes, (Aim 2) defining their respective genetic and nutrient dependencies and (Aim 3) using these insights to manipulate adaptive pathways for cerebral hypoxia in a mouse model of stroke. We hypothesize that unique metabolic pathways underlie the differences in ischemia sensitivity of neurons vs. skeletal myotubes. By understanding such differences, we hope to uncover novel hypoxia adaptations and apply them to disorders of impaired oxygenation such as ischemic stroke.